bpf: Fix propagation of signed bounds from 64-bit min/max into 32-bit.

Similar to unsigned bounds propagation fix signed bounds.
The 'Fixes' tag is a hint. There is no security bug here.
The verifier was too conservative.

Fixes: 3f50f13 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211101222153.78759-2-alexei.starovoitov@gmail.com

kernel/bpf/verifier.c

@@ -1420,7 +1420,7 @@ static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
 
 static bool __reg64_bound_s32(s64 a)
 {
-	return a > S32_MIN && a < S32_MAX;
+	return a >= S32_MIN && a <= S32_MAX;
 }
 
 static bool __reg64_bound_u32(u64 a)